I'm using a $700 Hsu 3.3 in a 12x16 room. Admittedly, I'm only using a fraction of its power but I mostly bought it for its bass extension. Unless you are planning to upgrade your sub soonish, I'd urge you to up your budget enough for one that at least goes down to around 20 Hz. That will cover all the important music and HT stuff. Some music goes lower and some HT effects go lower but if the sub plays at 20 Hz within about 6dB you probably won't miss much.
Jim